Hi everyone, my name is Jim. I am planning to build an RC sub of my own design, completely from scratch. I've been reading a lot about the concept online and developed my plan a bit. I want to start with the Water Tight Compartment and have a few questions if you don't mind:

1. I've seen references to the WTC remaining sealed for lengthy periods. If it is sealed most of the time, where does the battery go? If inside the WTC, how is it charged without breaking the seal? If outside the WTC, how is it protected from water? Or is the WTC meant to be opened and closed every time the battery needs to be recharged?

2. With the electric motor (I'm planning to use an old RC aircraft motor) inside the WTC, what kind of seal is commonly used around the drive shaft as it leaves the WTC? Bicycle bellows or the shaft through a rubber block with grease inside it? If a bellows, how does this mate to the spinning shaft?

3. I believe the MHz range radio/receivers are best for penetration of water by the signal but these have a long aerial on the receiver. How is the aerial usually arranged inside the sub - all bundled into the WTC or does it extend outside the WTC so it can be straight?

4. How do you water-proof the exit point from the WTC for the wires leading to the servos for the planes?

1. I've seen references to the WTC remaining sealed for lengthy periods. If it is sealed most of the time, where does the battery go? If inside the WTC, how is it charged without breaking the seal? If outside the WTC, how is it protected from water? Or is the WTC meant to be opened and closed every time the battery needs to be recharged?

Depends on the layout of the boat and personal preference. Batteries situated externally work well, so do internally mounted ones. Lipos are best run inside the tube however. If the batteries are internal, then you have to open it up each time you charge the batteries. Don't ever charge batteries inside a sealed WTC, that combination makes a potential DIY grenade.

2. With the electric motor (I'm planning to use an old RC aircraft motor) inside the WTC, what kind of seal is commonly used around the drive shaft as it leaves the WTC? Bicycle bellows or the shaft through a rubber block with grease inside it? If a bellows, how does this mate to the spinning shaft?

A simmerring (miniature oil seal) or o-ring within a compression gland is usual. Commercial units are available to purchase if you can't fashion your own.

3. I believe the MHz range radio/receivers are best for penetration of water by the signal but these have a long aerial on the receiver. How is the aerial usually arranged inside the sub - all bundled into the WTC or does it extend outside the WTC so it can be straight?

Correct. the modern ghz sets are unfortunately hopeless underwater.

Best to run the aerial as long and straight as possible, preferably in the wet for optimum range. Not always possible however, so sometimes it's coiled up inside the WTC.

4. How do you water-proof the exit point from the WTC for the wires leading to the servos for the planes?

Rubber bellows or control rod seals- o-ring or lip seal based.

5. Plane servos - how are the servos, themselves, water-proofed?

They go inside the WTC.

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JimRC wrote:1. I've seen references to the WTC remaining sealed for lengthy periods. If it is sealed most of the time, where does the battery go? If inside the WTC, how is it charged without breaking the seal? If outside the WTC, how is it protected from water? Or is the WTC meant to be opened and closed every time the battery needs to be recharged?

The battery can be located within the dive module or out in the 'wet' depending on the battery type and your preference. For example the sealed lead-acid gel cells work perfectly fine being in the water. Just be sure to insulate the connections ( e.g. with silicon sealant) so that you do not get galvanic corrosion of the connectors. Some folks, including myself, actually place the batteries in their own 'dive module'. This is very useful for using other battery technologies like NiMH or LiPo batteries which don't really like to get wet.All that said, there is nothing wrong with placing the battery within the dive module, but you will want to either remove the battery for charging or at least have a way to vent air through the module so as to prevent a hydrogen build-up.

JimRC wrote:2. With the electric motor (I'm planning to use an old RC aircraft motor) inside the WTC, what kind of seal is commonly used around the drive shaft as it leaves the WTC? Bicycle bellows or the shaft through a rubber block with grease inside it? If a bellows, how does this mate to the spinning shaft?

As you surmised all moving parts (e.g. prop shafts and servo rods) that need to exit the dive module need a form of seal. The most common type is an o-ring or a cup seal. Check out Mike's Subworks to see what one of these shaft seals look like.

JimRC wrote:3. I believe the MHz range radio/receivers are best for penetration of water by the signal but these have a long aerial on the receiver. How is the aerial usually arranged inside the sub - all bundled into the WTC or does it extend outside the WTC so it can be straight?

Yup, you'll need to use a radio in the 27 - 75MHz range. The actual frequency depends on which country you are in. (In the U.S. the 75MHz frequency is reserved for surface craft)Running the aerial can be done one of two ways: just run it back-and-forth within the dive module or run it out the aft (or forward) bulkhead. If you do the latter, most folks run a piece of threaded rod through the bulkhead to act as a waterproof connector. Make sure that the wire that is outside the dive module and in the water is completely sealed at both ends otherwise you'll just ground out the antenna. Next you'll need to make sure that the total length of the antenna remains the same....so you will need to cut the receiver antenna to remove a length equal to the length of the wire outside the dive module.

JimRC wrote:4. How do you water-proof the exit point from the WTC for the wires leading to the servos for the planes?

Most everyone place the servos within the dive module and run the servo rods through a bulkhead seal. (See answer to question #2)